optimization; rewrite the do_expand function in order to:

  + give it a better interface
  + get a rid of most of the fixed stack buffers and then
  + no longer limit rewritten uris, expanded variables etc to
    2k
  + make it better readable and understandable at all.

This is a two-tiered commit for better diffs.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@100675 13f79535-47bb-0310-9956-ffa450edef68